We live in an age of great scientific and technological innovation, and what seemed out of the question or at least very doubtful only a few years ago, today lies almost within our grasp. In no area is this more true than that of human health care, where lifesaving and life-enhancing technologies have given, or have the enormous potential in the not so distant future to give, relief from some of the most terrible human illnesses. On two fronts in particular, xenograft or cross-species transplantation and genetic engineering of animals on behalf of gene therapy in humans, such relief appears very promising, if not actually on the horizon. Certainly, extensive research work on both fronts is underway both in the United States and abroad